This type of motion guide apparatus includes a track rail and a block assembled via a rolling element to the track rail in such a manner as to be movable in a length direction of the track rail. A rolling element rolling portion extending in the length direction is formed on the track rail. A loaded rolling element rolling portion facing the rolling element rolling portion of the track rail is formed on the block. A plurality of rolling elements is placed between the rolling element rolling portion of the track rail and the loaded rolling element rolling portion of the block in such a manner as to be capable of rolling motion. When the block moves relatively to the track rail in the length direction, the rolling elements between the block and the track rail perform rolling motion. Light movement of the movable member such as a table attached to the block can be obtained by using the rolling motion of the rolling elements.
A rolling element retainer to prevent the rolling elements from falling off is incorporated in the block (refer to Patent Literature 1). The rolling element retainer includes retaining portions extending in the length direction along the loaded rolling element rolling portion of the block. If the rolling element is a ball, the width of an opening between the retaining portions is set to be smaller than the diameter of the ball. The rolling element retainer holds the rolling element with the retaining portions and accordingly prevents the rolling element from falling out of the block.